I am trying to design a new home. We had a great ranch with walkout basement plan, but it has been quoted at 50% higher than our budget. We took a close look at our priorities and decided we could live without most of what was designed into the basement. So now I have a one level plan with everything I need. It's about 2000 square feet. We like ICF for wind resistance (since there will be no basement to hide in during storms). We live in SE Minnesota, so appreciate the high R value the ICF wall provides. So we are keeping the ICF in this one level plan. ICF will provide the frost wall down four feet to a foundation, and continue to the roof eave. I assume a cement slab floor, with 3 inches foam board insulation and vapor barrier on top of foam under slab. Attic will have 2 inches of spray foam, followed by R50 blown fiberglass, inside energy heel trusses. I have hydronic radiant floor heat experience, but need air conditioning. If I want good indoor air quality, is my only choice to use a fully ducted HVAC system? If so, how do I run ducts? There is no floor joist area to run the trunks in, since the floor is concrete. Do they put runs in the sand below the concrete? Seems difficult to do that properly. Can the runs be put in the attic if they are insulated? How is that done? Is the concret slab floor ever poured at different levels to allow all the floor covering types to end up approximately at the same elevation? Thanks.

Posted By jimmyge on 29 May 2015 10:51 PM

If I want good indoor air quality, is my only choice to use a fully ducted HVAC system? If so, how do I run ducts? There is no floor joist area to run the trunks in, since the floor is concrete. Do they put runs in the sand below the concrete? Seems difficult to do that properly. Can the runs be put in the attic if they are insulated? How is that done? Is the concret slab floor ever poured at different levels to allow all the floor covering types to end up approximately at the same elevation? Thanks.

I would look into ductless mini splits like the Mitsubishi units. The new 3/4 and 1 ton units get 30 SEER and price out to around $3k installed. No need to run inefficient and costly duct work.

If you have an open floor plan and/or leave the doors mostly open, you can get by with just a few units depending on the size and layout of the home.

We used ductless mini-splits with excellent results here in SC. If you want to go with a ducted system, the ductwork should be in the conditioned space, BELOW the attic insulation. You can either use various soffits to hide the ductwork, or have a dropped ceiling with space above for ductwork (and plumbing and wiring, too.) You might find that your house doesn't need a very big A/C unit, especially in Minnesota. Our cooling load on 1400 square feet of very similar construction is only 9000 BTU, and I'm sure it's hotter in SC. A central system might be too big to run efficiently. If the system is too big, not only will it cost more to run, but it won't dehumidify correctly, which is often more important for comfort. You need to run an accurate Manual J to see how much cooling you need.

You don't say what your wall R value is, or the value of your windows. If you build a good envelope, and your slab and attic are a good start, 2-3 mini splits would work very well for both AC and heat. With a tight house, you should always install a heat recovery ventilation system which will need some ductwork which can be run easily in dropped soffits. These systems, along with the mini splits will give you excellent indoor air quality. To avoid ductwork, use the Lunos system: http://foursevenfive.com/product-category/ventilation/lunos-e2-2/

thanks for the replies. Are you suggesting to go ahead with HRFH, and use the mini splits for AC , and optionally get some air source heat from the mini splits in mild heating season? If so, then do I have a HRV plumbed in seperately? It seems to be code now to use some sort of HX, as the code requires "a balanced fresh air system". Typically where would the HX plumbing be? Seems the easiest choice is in the attic if it is insulated where necessary.

what is a HRHF?
You can use mini splits for heat, but you'll need to supplement it when temps get below -20.
There is ductwork for an HRV, but no plumbing is involved.
All the ductwork for your systems MUST be within the thermal envelope. The Only thing in your attic should be insulation.

Thanks Bob. Our posts crossed. The walls are ICF, so 2.5 inches foam on outside and 2.5 on inside of 6 inches of poured concrete. That is EPS, totaling 5 inches, which to me means about R20. But somehow they claim it is approx R40. HRFH is hydronic radiant floor heat.

Despite "their claims", R-20 is not enough. Build a 2x6 wall inside the ICF's and fill it with R20 insulation. This is not a structural wall so will be pretty inexpensive to build and can be used as a wiring chase.

Posted By jimmyge on 30 May 2015 08:37 AM
Thanks Bob. Our posts crossed. The walls are ICF, so 2.5 inches foam on outside and 2.5 on inside of 6 inches of poured concrete. That is EPS, totaling 5 inches, which to me means about R20. But somehow they claim it is approx R40. HRFH is hydronic radiant floor heat.

Some ICF brands can have much thicker foam on any side desired to increase R-value. I think in your area, it would be best to place the extra foam outside the concrete. Think thermal mass.
 
I, too, suggest that you consider mini-splits for heating and cooling. However, if you must use ducts, then consider roof trusses that create a chase above the ceiling that can be insulated in the attic. As mentioned by another poster, the ducts can also be placed below the ceiling in a hallway chase so that the attic insulation will insulate the ducts.  This approach will reduce the height of the ceiling in the hallway. 

If the attic insulation is as thick as you mentioned, then consider installing a catwalk for access to all parts of the attic. Otherwise, trips in the attic will tromp down the insulation.  Catwalks can also be used for limited storage.  Also to look for roof leaks.

then consider roof trusses that create a chase above the ceiling that can be insulated in the attic.

And make sure that these chases are vented to the interior. Ie, heat/pressure/humidity-wise, the ducts are inside the envelope.

Posted By Bob I on 30 May 2015 08:54 AM
Despite "their claims", R-20 is not enough. Build a icki2x6 wall inside the ICF's and fill it with R20 insulation. This is not a structural wall so will be pretty inexpensive to build and can be used as a wiring chase.

I have never talked to an ICF home owner that has added insulation to the basic wall. If i felt i needed more, it would sicken me to have to build a house with walls nearly 20 inches thick (11 inches of ICF plus a 2x6 plus sheet rock and siding). The taxman sure would appreciate it though.

In cases where more insulation is desired, I think it makes more sense to add EPS outside the concrete than to build a stud wall inside the home. As I said earlier, some ICF forms can include thicker EPS.

So, learning about your recommended mini splits. ... Does a home require something in every room? Like a bathroom, do I put an indoor unit in the bathroom? If not, do these neglected rooms get kind of stale? Do some large rooms (like 500 square feet) require two indoor units to evenly serve? If i build the new house using only minisplits, I'm afraid I'd miss the constant recirculation and filtration of my ducted system I am used to in my current house. Thanks guys!

IMO, if you want a concrete wall and more than R20 or so, don't use ICFs or foam. Just pour concrete, build a second wall with studs and fill the gap (any size you want) with cellulose.

You can't get R20 (whole wall) with studs and cellulose unless you add 1" or more of foam to the face of the studs (2x6) or go with studs wider than 2x6. A 2x6 framed wall filled with cellulose will only give you ~R15 whole wall R value at 16" on-center or ~R17 whole wall at 24" on-center when you factor in the R value of the wood.

Our ICF walls are Fox Blocks, rated at R-24. The thing with ICF is that there is little or no thermal bridging, and the method is inherently well-sealed, so you get a true R-24. They do sometimes claim much higher R-value, and under certain conditions, they do perform at much higher levels. We have someone on here that did seasonal studies, and the ICF sometimes performed as if much higher. I'm not sure of the physics involved. In our house, with R-50 attic, the temperature is very, very even throughout. We normally run one single output mini, and keep all of the interior doors open. I have found the temperature to be within about 1 degree F. throughout. For some reason, though, the humidity is a bit higher in the farther rooms. It's hard to express how comfortable this house is, but then, I'm in my 60's, and this is the first insulated house I've lived in, despite having lived in CT and FL. A properly designed HRV (which you will need) will also help circulate air. Another option is 'ducted' minis, which can serve more than one room with one head. I couldn't get the efficiency and low temperature performance I wanted with a ducted or multi-head unit, though.

Thanks, I really appreciate your experience with this type of home.

"it would sicken me to have to build a house with walls nearly 20 inches thick"
Au Contrair: We build houses with 12" walls as do dozens of builders around the country, and contrary to your statement, those homes, with no fossil fuels being burned and with a fresh air ventilation system, have extremely good air quality. People building tight houses without ventilation systems are, as you suggest, subject to unhealthiness.

"Does a home require something in every room?"
Yes and No. We recently completed a 3300 Sf 10 room home which is comfortably heated and cooled with four mini splits. It does have fresh air ventilation in all living spaces and exhaust vents in all baths, laundry and kitchen. The key is that the house is tight and super insulated so the heat loss from each individual room - even corner rooms on the north & west side - is very small, so normal, unducted air circulation is enough.

We use a double stud cellulose filled wall; two 2x4 walls with a 5" space between. Almost any kind of insulation will work, and in tornado areas ICFs are probably worth the extra cost, but you should not trade the insulation value for the type of insulation.

Rather than a slab, why not build a conditioned crawl space and put the duct work down there? Your foundation walls will be insulated with the ICF. You just need to put 2-3 inches of foam on the ground under the crawl space, then pour a thin 2" or so thick "rat slab" to seal it up. You could also coat the inside of the ICF foundation walls with a cement based plaster and use the entire crawl space as a large return air plenum. If you have sufficient height under the floor system, you could place the HVAC unit down there as well.

Have you considered geothermal heating/cooling? With geothermal, you would have one system to provide all your heating and cooling needs, and most of your hot water needs assuming you install a unit with a hot water generator and unpowered pre-heat storage tank (recommended). It generally has a high up front cost, but there is still a 30% federal tax credit in effect until the end of 2016.

Both geothermal and mini splits are heat pumps; both do heating and cooling. The advantage of mini splits is that they are typically much less expensive to install; the advantage of geothermal is that there is no cold shut off, so they still work when it gets to -30. Both are far less expensive to run than fossil fuel units and both can be fueled by energy which you can generate yourself with no fuel cost. And both (in at least some areas) generate some rebates.